Evidence is presented for the presence of an intracellular iron transport agent in rabbit reticulocytes. By isolating ghosts from whole reticulocytes which had been incubated with 59Fe3 plus or minus transferrin it is possible to obtain plasma membranes with specifically bound radioactive iron. The mobilization of iron from these membranes is greatly enhanced by fresh lysate and by ATP. The iron that is released is in the form of a low molecular weight iron-protein complex. The iron of this complex is readily donated for hemoglobin and ferritin production. The protein has been named erythroferrin. The proposed research centers about the characterization of erythroferrin and its cellular role. In particular, a careful study of the physical and chemical properties of erythroferrin will be made including molecular weight, amino acid analysis, number of irons per mole, and oxidation state of the iron. The ability of erythroferrin to donate iron to protoporphyrin and to exchange iron with ferritin will be examined. The mechanisms by which erythroferrin mobilizes iron from the membrane will be investigated, with special emphasis on cofactor involvement. The information gained will contribute to the knowledge of red blood cell iron metabolism and the integration of iron into hemoglobin synthesis.